Outboard motor



Ihwentor attornegs F. T. IRGENS OUTBOARD MOTOR Aug. 21, 1951 Filed Jan. 5 1950 F'I/VN T Leas/v.5 M, A ms;

Patented Aug. 21, 1951 UNITED STATES PATENT OFFICE OUTBOARD MOTOR Finn T. Irgens, wauwatosa, Wis, assignor to Outboard, Marine "& Manufacturing Company, Waukegan, Ill., a corporation of Illinois Application January 3, 1950, Serial No. 136,577

1 This invention relates to improvements in outboard motors. I f

It has long been the practicev to provide conventional outward motors having upright drive shafts with streamlined shaft housings known to the trade as lower units on which anticavitation plates have been disposed horizontally above the path ofpropeller blades. The purpose of such a plate is to'minimize the possibility that air may be drawn from the surface into the propeller, thereby reducing propeller thrust. The desired drive shaft position'in such an outboard motor is usually vertical but, regardless of whether the drive shaft has been intended to be vertical or slightly inclined, the design' has always been such that the anti-cavitation plate has been horizontal in use, in order to bring about a minimum displacement of the water through which the plate passes.

The present invention is based upon the discovery that for certain purposes it is advantageous to have the plate, or at least the lower surface thereof, disposed obliquely at a slight upward angle which, in practice, may be of the approximate order of 4 degrees, measured with the boat at rest. Since the propeller shaft is ordinarily designed to be horizontal in use, the anti-cavitation plate is made to diverge forwardly from the axis of the propeller shaft in an upward direction at about the specified angle. It should a further be explained that the anti-cavitation plate referred to is not a surface plate but normally operates slightly submerged in order to perform its anti-cavitation function. Thus, in-

stead of lying in a plane which is normal to the axis of the upright drive shaft, the anticavitation plate of the present invention, or at least the lower surface thereof, lies in a plane which intersects the axis of the propeller shaft at an angle of 94 degrees, or approximately that figure.

The objects of this invention are to promote the better action of the anti-cavitation plate by tending to thrust downwardly upon the water in an area overlying the path of the propeller and thereby to preclude the formation of flow pressure areas into which air or exhaust gases could be sucked causing cavitation; and to exert lift during operation of the outboard motor which tends to support not only the motor but the boat on which it is mounted, to prevent the boat from settling at the rear either as a result of the weight of the motor or as a result of propeller action. In the drawings:

t 6 Claims. (Cl. -17) Fig. 1 is a view in side elevation of an outboard motor embodying the invention, portions thereof being broken away sufficiently to show the under water exhaust outlet.

Fig. 2 is a view taken in section substantially at the water line on the line 22 of Fig. 1.

Fig. 3 is a fragmentary side elevation of the lower unit of an outboard motor showing a slightly modified embodiment of the invention.

The outboard motor shown in Fig. 1 is of generally conventional design having a mounting bracket 4 for connection with a boattransom. The mounting bracket supports a swivel bearing 5 in which the shaft housing 6 is swiveled. At the upper end of the shaft housing is the powerhead I and at the lower end is the gear housing or lower unit 8 which supports the propeller shaft 9 on which propeller I0 is mounted. The drive shaft II is connected to the propeller shaft 9 by means of the usual bevel gearing at l2.

Carried by the lower unit or gear housing 8 at a point somewhat below the water line is the anti-cavitation plate I5 which is usually cast integrally with the lower unit or some portion thereof. In the embodiment shown in Figs. 1 and 2, the anti-cavitation plate l5 has its upper surface l6 horizontal when the outboard motor is in a position for use, the lower surface I! of the anti-cavitation plate being inclined with its forward end higher than its rearward end. As a result, .the anti-cavitation plate has the aspect in side elevation of an elongated tapered wedge, as clearly appears in Fig. 1.

The increased thickness of the anti-cavitation plate at its rear end is taken advantage of to provide an exhaust outlet at I8 with which communicates the exhaust chamber IS in the lower unit, to which the exhaust pipe 20 leads from the muffler 2|.

As explained above, the outboard motor is ordinarily mounted on the boat in a position, as determined by the adjustable thrust member 22, so that the drive shaft I l is substantially vertical and the propeller shaft 9 substantially horizontal. It is not essential, however, that the shafts be vertical or horizontal respectively. In the particular embodiment shown in Fig. 1 and Fig. 2, the upper surface l6 of the anti-cavitation plate is parallel to the axis of the propeller shaft and normal to the axis of the drive shaft H, being substantially horizontal and uniformly spaced below' the surface of the water. The water level may be assumed to be approximately at the point indicated by the section line 2-3:

The upward inclination of a lower surface portion I! of the anti-cavitation plate in a forward direction will tend to cause a planing action when the outboard motor is in operation on a boat. It is not necessary that the whole of the lower surface be inclined to get this result. The advantage of the inclination is to contribute lift at the stern of the boat. Most boats which have outboard motors attached tend to be heavy at the stern, partly by reason of the weight of the cut board motor itself; partly because, if the boat has a single occupant, he usually sits in the stern in order to control the motor; and partly because the slip stream of the propeller tends to draw water from beneath the stern of the boat, thereby lowering the stern. This latter phenomenon is, of course, common in many boats other than those powered with outboard motors.

The inclined plane provided at I! tends, during operation, to raise the outboard motor in the water through which it passes. This lifting tendency is necessarily communicated to the boat, since the vertical thrust developed at the lower unit is so close to the usual pintle 23 upon which the outboard motor tilts, that virtually no tilting moment is developed and certainly not enough to overcome the forward thrust of the propeller in an opposite direction. Thus, the boat tends to ride more nearly level in the water with its propeller axis more nearly in the desired direction of thrust.

- In addition, it seems to be possible to make the anti-cavitation plate smaller when it is set at a slight angle. This is contrary to previously accepted principles, since every effort has been made to avoid creating any disturbance in the water and it has heretofore been supposed that setting the plate,'or its lower surface, at an angle to the direction of travel would bring about the very kind of disturbance which would induce the cavitation disturbance which it is the function of the plate to control. In connection with the present invention, however, it is found that the tendency of the propeller to cavitate is further reduced by a plate design which tends to establish a high pressure area over the propeller, as is done where at least a substantial portion of the lower surface of the plate is inclined upwardly and forwardly, as shown. This tends to maintain solid water, uncavitated by air, for the propeller blades to act upon in the course of their ad- Vance.

Despite the resulting thickness of the plate at its rear margin, substantially no vortex is created at this point because of the delivery of exhaust gases into what might otherwise be a void behind the plate. It is not new to deliver exhaust gases through the rear of an anti-cavitation plate but, in previous designs, the thickness of the plate for accommodating the exhaust gas outlet has been provided by a symmetrical expansion of the plate above and below its center line, whereas in the present design, the entire thickness, or substantially the entire thickness, is provided by the plate form which establishes the inclined plane along the lower surface of the plate.

Fig. 3 shows a modified embodiment wherein the exhaust pipe 2:; leads into a lower unit Bil which has a downwardly and rearwardly inclined pipe-like exhaust discharge spout at 24. construction, the anti-cavitation plate we need not have any greater thickness than is required for physical strength and it may be of substantially uniform thickness, as shown, both the upper surface I60 and the lower surface H being In this at the same inclination. As in the previous de sign, the angle of inclination to the horizontal may range approximately from three to six degrees, the angularity shown being four degrees. As shown, the drive shaft H is vertical and the axis upon which the propeller is rotatable is horizontal. Assuming that these angles may be varied in practice, the outboard motor will, in all instances, be so set with respect to the boat upon which it is so supported, that in use the anticavitation plate made in accordance with the present invention has at least its lower surface constituting an inclined plane sloping upwardly in a forward direction so as to exert lift on the rear of the boat to which the motor is attached in operation.

I claim:

1. An outboard motor lower unit of the type having a generally upright drive shaft and a propeller shaft, gearing connecting said shafts, and a housing for said shafts and gearing, said lower unit being provided with an anti-cavitation plate on said housing having its upper surface substantially horizontal and at least a substantial portion of its lower surface upwardly inclined in a forward direction at a slight angle to the horizontal, the rear end of the lower surface of the plate being lower than the forward end of such surface.

2. The device of claim 1 in which the upward inclination of said lower surface is of the approximate order of four degrees to the horizontal.

3. For use in an outboard motor of the type having a shaft housing with a power head at its upper end and a lower propeller-carrying unit at its lower end and means for mountin the shaft housing on a boat to operate at a normally fixed angle respecting the boat; a lower unit having an integral anti-cavitation plate with its upper surface substantially horizontal and only its lower surface inclined upwardly in a forward direction for exertin downward pressure on the water which it traverses and developing an upward reaction lift for transmission through said shaft housing to the boat with which such shaft housing is connected.

4. The device of claim 3 in which said plate is wedge-shaped and has its lower surface at an inclination of approximately 4 to the upper surface and is provided at its rear end with an exhaust discharge port for which said lower unit has a communicating passage.

5. For use in an outboard motor of the type having a shaft housing with an internal combustion engine at its upper end and a lower propellercarrying unit at its lower end and means for mounting the shaft housing on a boat; a lower unit comprising unitaril a gear casing and an anti-cavitation plate mounted thereon and wedgeshaped in side elevation with its apex end directed forwardly and a thicker end extending rearwardly, the lower surface of said wedgeshaped plate being inclined upwardly in a forward direction and said plate having an exhaust passage extending therethrough and opening dan internal combustion engine having a generally upright drive shaft, a housing for said drive shaft with which said engine is connected, a propeller gear casing connected at the lower end of the housing, a propeller shaft'mounted in said casing and. carrying a propeller at the rear of the casing, gearing in the casing connecting the drive shaft with the propeller shaft, and an anti-cavitation plate mounted on the drive shaft housing and overlying the path of movement of the propeller, said anti-cavitation plate being wedgeshaped in side elevation with a forward apex and a rear end, the said end being provided with a port disposed rearwardly of the propeller and above the lower surface of said plate, and with which port said engine has an exhaust gas connection, the said lower plate surface being downwardly inclined from said apex to said rear end and the combined effect of the wedge-shaped form of said plate and the inclination of its lower sur- 2o REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,734,911 King Nov. 5, 1929 2,259,153 Conover Oct. 14, 1941 FOREIGN PATENTS Number Country Date 1,563 Great Britain of 1892 458,770 Great Britain Dec. 28, 1936 

